Processes in which modulated pulses are used to control power transistors are referred to frequently as pulse duration or pulse width modulation (PWM) systems and can be used to control alternating current motors. In pulse width modulation, a control unit determines a duration of each of a succession of pulses which are applied as voltage pulses to control power transistors so that behind the power transistors, a correspondingly pulsed supply voltage is developed across the motor winding.
Since the motor winding has substantial inductivity, the high frequency alternating voltage can be smoothed to a more or less direct current. If the inductivity of the windings themselves are insufficient for this purposes, ahead of the motor terminals, a smoothing filter can be provided. The result is a direct current flow through the windings or a direct voltage applied across the winding. This direct current can be varied as to its level by modulation of the pulse width and the motor can be a dc motor.
The modulation of the pulse width can be so effected that in the motor windings an alternating current of comparatively low frequency can flow.
The pulse width modulation is effected utilizing a microprocessor which is connected to a module for generating the control signals for the power transistors (PWM module) by writing the pulse width of the next pulse to be generated as a value in a register. The PWM modules reads this value out and switches the power transistor on for the duration of the read value. If, based upon such PWM values, it is desirable to generate a curve, for example, in alternating voltage curve, the microprocessor must inscribe for each switching time a respective value in the register. As a result, a relatively large processor capacity or computer capacity is required so that the microprocessor can respond rapidly to load limits and the like. As soon as the limit is exceeded and the processor is no longer capable of resetting the register in the correct rhythm, the output of the PWM module and the resulting low frequency alternating current signal contain discontinuities. These can lead to electromagnetic failures or noise and require larger filters or additional filter components at considerable cost.